First-order sapphire fiber Bragg gratings for high temperature sensing

in: Temporal Proceedings (2016)
Elsmann, Tino; Habisreuther, Tobias; Graf, Albrecht; Rothhardt, Manfred; Schmidt, Markus A.; Bartelt, Hartmut
Sapphire fiber Bragg gratings (SFBG) are presented as a new kind of high temperature sensor that isnot sensitive to electromagnetic fields and can be operated in air as well as inert atmosphere. Forinscription of first order gratings femtosecond laser pulses with a wavelength of 400 nm are applied.Multiplexing of several gratings within one single crystal fiber is demonstrated. The gratings weretested for short term up to 1,900°C and for a period of 28 days at 1,400°C. An adapted signalprocessing of the reflexion spectra of the highly multimode air clad sapphire fibers with a diameter of100μm allows detecting relative temperature changes better than ±1K. Tubes can protect the air cladsapphire fiber from reactions with the environment. Applying a bare fiber allows to detect temperaturechanges with several Hz speed. Strain sensor designs and strain measurements based on SFBG forapplications up to 600°C were tested, too. The sensitivity of multimode SFBG strain sensors is Δl / l =10-5 (10μstrain). Overall, fiber Bragg grating inside Sapphire fibers provide a new base for precisehigh-temperature sensors with key advantages such as signal multiplexing, large temperaturebandwidth and insensitivity to electromagnetic fields.

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